The present invention pertains to hydraulic control systems and is directed more specifically to such a system for a vehicle of the class propelled by a pair of hydraulic bidirectional motors and also having several hydraulic actuators for manipulation of an implement.
Some off-highway self-propelled work machines operate hydraulically. A typical example of such hydraulic vehicles comprises a pair of travel motors and a plurality of implement actuators such as an arm cylinder, boom cylinders, and a bucket cylinder, as well as a swing motor for revolving the superstructure of the vehicle. The travel motors are powered respectively by a pair of pumps via a pair of travel valves. The implement actuators, including the swing motor, are also activated by the pumps via two groups of implement valves. Each group of implement valves, associated with one pump, are in parallel connection with one another and with one of the travel valves.
A problem has existed with this type of hydraulic vehicle control system. Let it be supposed that the vehicle travels forwardly or rearwardly, with both travel motors set into rotation in one and the same direction, while any of the implement actuators are being powered by one of the pumps. In that case this one pump must feed both the implement actuator and one of the travel motors whereas the other pump is required to feed only the other travel motor. The consequent difference between the rates of delivery of the pressurized fluid from the pumps to the travel motors has rendered the straight travel of the vehicle difficult or impossible.